1. Field of the Invention
The present invention relates to a range finding device of the so-called passive external light type which detects the distance to an object using an object image obtained by receiving the reflected external light (peripheral light) from the object.
2. Description of the Related Art
Fixed lens cameras such as compact cameras and so-called lens shutter cameras and the like are provided with a range finder of the so-called passive external light type which comprises, adjacent to a view finder optical system, a pair of line sensors, and a distance objective lens to form on the line sensors an object image included within the range frame provided within the view finder.
The ranging principle of the aforesaid range finder is the detection of the amount of relative positional dislocation of an image sensed by one line sensor relative to an image sensed by the other line sensor, and calculating the object distance using the amount of dislocation.
FIG. 10 illustrates the ranging principle of a conventional range finder of the passive external light type.
Within range finder 20 are provided an autofocus (AF) sensor comprising a pair of charge-coupled device (CCD) line sensors 21 and 22 to photoelectrically convert a sensed object image to electric signals, and lenses 23 and 24 to form the object image on the CCD line sensors 21 and 22. The number of pixels M of the image sensing range of CCD line sensor 22 are a predetermined number r in excess of the number of pixels N of the image sensing range of CCD line sensor 21.
CCD line sensors 21 and 22 are arranged a predetermined distance XO on the same line.
When a partial image having the same number of pixels as the number of pixels N comprising an image GL sensed by the CCD line sensor 21 is extracted from the image GR having M pixels sensed by CCD line sensor 22 as a second image for use in comparison, a second image GR(i) of (r+1) is obtained (where i=0, 1, . . . r). When the left edge of image GR is used as a reference position, a partial image having N pixels is extracted from each second image GR(i) by shifting i pixels from the reference position toward the right side.
Then, the object distance is calculated by comparing the first image GL and the second image GR(i) of (r+1), calculating a second image GR(i) matching both images, and calculating the amount of dislocation Xd of the second image GR(i) relative to the first image GL. That is, when an object is at an infinity position, range finder 20 makes the adjustment so that first image GL matches the second image GR(0), and the second image GR(i) matching the first image GL is shifted to the right side in accordance with bringing the object nearer from the infinity position. Therefore, when the number k second image GR(k) matches the first image GL and the distance from the reference position at image GR of the second image GR(k) is designated Xi, the amount of shift Xd of the first image GL and second image GR(k) is calculated by the expression (XO+Xi). Since the object distance D0 corresponding to the infinity position is preset, the object distance D0, the object distance Dk is calculated using the distance X0 between the first image GL and the second image GR(0), and the amount of shift Xd between the first image GL and the second image GR(k).
The second image GR(k) which matches the first image GL is determined by calculating the correlation value S(i) that expresses the degree of matching of each second image GR(i) relative to a first image GL. As shown in FIG. 11, the correlation value S(i) is defined by the total sum of the absolute value of the level differential of the pixel data g1(j) comprising the first image GL and the pixel data g2(j+k) comprising the second image GR(k) at a position corresponding to the pixel data g1(j) (i.e., the j position) as shown below; EQU .SIGMA..vertline.g2(j+k)-g1(j).vertline.
when the position of the second image GR(k) is expressed by the shift position from a reference position of the first image GL, the second image GR(k) having a minimum value Sm of a waveform in a correlation series of correlation values S(i) corresponding to the shift position i is calculated as the second image matching the first image GL, as shown in FIG. 12.
The range finder 20 is capable of measuring to a range closer than the closest distance of the object distance range measurable by the autofocus system of the camera (hereinafter referred to as "AF controllable range"); FIG. 12 shows the shift range wl expressing the range of detectable object distances. The close shift range w2 expresses a range corresponding to object distance (near range W in FIG. 1) closer than the aforesaid AF controllable range.
In conventional cameras provided with passive external light type rangefinders, when the minimum value Sm of the waveform correlation series is detected by the close shift range w2, AF control is not possible and a warning hereinafter referred to as "proximity warning") is issued expressing that the object is closer than the AF controllable range by having, for example, the LED display flash to indicate the focus state in AF control. When the minimum value Sm of the waveform correlation series cannot be detected, release prohibited, impaired ranging and like warning displays of predetermined impaired ranging processes are executed.
The aforesaid proximity warning alerts the photographer to the fact that an object is closer than the AF controllable range, and since AF control can be accomplished by setting a suitable object distance, it is desirable that the proximity warning is used as far as possible when the minimum value Sm of the correlation value series is in the close range w2 and outside the close shift range w2.
Conventional rangefinders execute impaired ranging processes without issuing a proximity warning when the minimum value Sm of the correlation value series is outside the close shift range w2 so as to similarly execute impaired ranging processes when the minimum value Sm of the correlation value series cannot be selected within the shift range w1.